EasyChemistryHigh School (9–10)GeneralThe Periodic Table › Development of the Periodic TableNumericalBoard Exam
The atomic masses of potassium and caesium are 39.1 u and 132.9 u…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
The atomic masses of potassium and caesium are and . Use Dobereiner's rule to estimate the atomic mass of rubidium, which lies between them. The real mass is . By what percentage of the real mass is your estimate off?
Need a reminder? The key ideaShow
Dobereiner: in a triad of three similar elements, the middle atomic mass is close to the average of the other two. Newlands: in order of atomic mass, every eighth element is like the first, which worked only up to calcium.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. Estimate. .
Step 2. Difference. .
Step 3. Error. .
Answer: , off by : potassium, rubidium and caesium make a good triad.
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